This research project is aimed at determining the functional role of certain proteins in vertebrate smooth muscle which have properties similar to those of some of the troponin subunits of vertebrate striated muscle. We aim to carry out a physicochemical characterization of these proteins and to study their ability to act as calcium regulators of the ATPase activity of actomyosin. We are also studying the possibility that these proteins may be able to act as calcium regulators of other enzyme systems in smooth muscle. We further intend to use immunolabelling techniques to localize these troponin-like proteins with respect to the contractile elements and other subcellular organelles in smooth muscle.